Even if a topic has since been deleted, you can get notifications for it. These include links to the topic page, like this, which give you a page with an unhelpful error message: "Insufficient permission to access the content. Return to MediaWiki. [0aa7d0a5] 2014-08-28 16:36:52: Fatal exception of type Flow\Exception\PermissionException"
Topic on Talk:Structured Discussions
Thanks for posting about this. Where was that notification -- in your Echo flyout, the All Notifications page, or in email? A deleted topic should be removed from Echo at least -- if you saw this notification there, we'll have to figure out what went wrong.
I put a card into the backlog to create a better error page, that at least has a link back to the board and/or board history. 1 Thanks!
DannyH (WMF): In Echo (and Special:Notifications). Interestingly, I only got the started a thread message, not the posted on a thread message.